This application claims the benefit of the Application Number 02 008 416.6 filed in the European Patent Office on Apr. 12, 2002.
1. Field of the Invention
The invention relates to a cutting device for bonded wires according to the precharacterizing clause of claim 1. In addition it relates to the use of such a device in a wire bonder, specifically a so-called thick-wire bonder.
2. Description of the Related Art
Cutting devices of this kind have long been known in wire bonders.
Wire bonders of many designs are in practical use, which produce a bonded contact by pressing a wire onto a substrate by means of a bonding tool (e.g., wedge) and using the tool to apply ultrasonic oscillations, causing the wire to become firmly fused to the substrate. For this purpose the bonding tool is disposed on a bonding head that is perpendicular to the substrate surface, and hence to the surface of the contact area for bonding, and can be moved up and down. To move the bonding head as required, suitable driving and drive-control means are provided. The substrate in turn is displaceable, in a plane substantially perpendicular to the direction of movement of the bonding head, so that bonded contacts can be produced at various places.
Unlike earlier bonding apparatus, which customarily comprises a plurality of drivers for the bonding head itself and the bonding tool, as well as in some cases for clamping devices to grip the wire to be bonded and for a knife to cut through the wire, the wire-bonder construction according to the applicant""s patent DE-U-93 21 269 introduced substantial simplifications and provided increased functional reliability and dimensional accuracy of the components being processed.
However, the working life of the knife used to cut the wire after production of the bonded connection has proved not yet to be satisfactory, and a need for improvement has also been discernible with respect to the quality of the cuts that allow the excess wire to be removed.
The objective of the invention is thus to provide an improved cutting device for bonded wire which, in particular, delivers qualitatively improved cuts along with an increased working life of the knife.
This objective is achieved by a wire-cutting device with the characteristics given in claim 1.
The invention includes the fundamental idea of a yielding or damped holder for the knife that cuts the bonded wire. This novel holder is intended to limit the peak forces that occur when the knife is applied to the bonded contactxe2x80x94in particular to the substrate after the bonded wire has been transected.
This limitation of peak force results, firstly, in clean cuts, and secondly it extends the life span of the knife by two- to three-fold, according to the inventor""s experience thus far. In addition, the risk of breakage is considerably reduced; knife fractures have practically been abolished with the new holder.
In a first preferred embodiment of the invention the knife is fastened to a holder with predetermined elasticity, which ensures that it will yield slightly in the direction of knife movement (i.e., substantially also the direction of movement of the bonding tool, which in the following will also be termed the Z direction). An elastic resilience in the range of a few hundredths of a millimeter is entirely adequate for the desired purpose; of course, the precise dimensioning of the holder should be adjusted according to the structure and intended application of the wire bonder.
In a more specialized embodiment this holder is designed to allow bending, in particular is provided with a flexibly bending connection between the knife and a force-transfer element that transmits the vertical movement component. In an especially simple embodiment this is achieved by a leaf-spring construction, into which there is inserted in particular at least one elastomer block for additional damping. Such a holder in some ways resembles the holder that attaches the bonding tool to the ultrasound exciter (transducer). In the inventor""s opinion at present, this holder is the preferred variant.
Alternatively, however, a seating that is elastic under compression can be provided, which for example, in a simple embodiment, comprises an elastomer element that is elastically distortable or compressible in the direction of the long axis of knife and force-transfer element.
Finally, the concept of the invention can be also be implemented with a hydraulic or pneumatic bearing of the knife on the piston of a piston-cylinder apparatus with a pressure-limiting element. The pressure-limiting element in particular is a simple pressure-relief valve.
In another preferred embodiment of the invention there is provided a means for adjusting the flexibility or the damping behaviour of the knife holder and/or a stopping device to limit the deflection of the knife owing to its ability to yield. Especially preferred in this regard is an adjustable stop, which is both responsible for limiting deflection and, when it is displaced, enables variation of the elasticity or damping properties, which are largely predetermined by the construction of the knife holder.
The flexibly mounted knife can be driven by a separate drive unitxe2x80x94however, it is more economical and simpler to provide a common drive for bonding tool and knife, i.e. for all movements of the relevant tools associated with the bonding head in the direction of the Z axis.
A particular aim of the invention is that the proposed cutting device for bonded wires also be applicable in a wire bonder wherein the knife is mounted to its bonding head with a predetermined degree of flexibility.